It is well known to incorporate coats in front surface films such as are used on displays, touch screens, windows, and other optical devices to protect the device and perhaps impart other desired benefits (e.g., privacy films, etc.). Many materials selected for such applications yield coats or films having smooth, glossy surfaces. Such surfaces tend to yield undesired glare due to external light reflected from the optical device, tending to interfere with desired use and performance of the optical device.
As a result, when used to form coats for front surface films, some coating materials are subjected to surface roughening or haze-inducing treatments to reduce the glare yielded by the resultant coat. In such antiglare films, scattering phenomenon (surface diffusion) of light caused by surface irregularity is utilized.
In some instances, sometimes in conjunction with surface treatments, antiglare films are also known in which particles having a refraction index different from that of a binder matrix are mixed into the binder matrix to impart internal scattering (internal diffusion) of light passing through the film caused by the difference in refraction indices of the binder matrix and particles and resultant optical interface.
U.S. Pat. No. 7,708,414 (Kameshima et al.) discloses an illustrative example of antiglare film comprising particles in a binder matrix.
Many conventional treatments for glare reduction, sometimes referred to as anti-glare (“AG”) treatments or coatings, yield films that exhibit small scale alterations of light and dark producing an undesirable, grainy, scintillating effect. This effect is commonly referred to as “sparkle”. The observed effect is highly angle-dependent such that minute changes in viewing angle produce an effect that is variously described as “scintillating”, “sparkly”, or “grainy noise”.
While we do not wish to be bound by this theory, FIG. 1 is a schematic diagram showing what is believed to be the mechanism for undesirable sparkle induced by a conventional AG film on an optical display. In this typical application, light emitting article 10 (e.g., an electronic display) comprises array 12 of light emitting pixels 14a, 14b, 14c, et seq. which emit light rays 16a, 16b, 16c, et seq. that are emitted from article 10 through face 19 of cover glass 18. To reduce glare, anti-glare film 20 comprising film 22 with anti-glare surface 24 has been adhered to face 19 with adhesive 26. Although film 20 serves to reduce the glare which would otherwise be generated by face 19, it does not transmit light rays 16a, 16b, 16c, et. seq. uniformly; instead emitting rays 28a, 28c relatively directly from rays 16a, 16c, respectively, while other rays such as rays 28b are emitted at substantially off axis orientation from display-emitted rays 16b. As a result, to an observer, rays 28a, 28c will be perceived as relatively bright portions of the image while rays 28b will be perceived as relatively dark. The impact will vary according to viewing angle, over small variations in perspective. The impact of such variation is perceived as sparkle.
In its most benign forms, sparkle may be merely a minor, aesthetic distraction. When more pronounced, however, it degrades resultant image quality, interfering with performance and utility of the optical device.
Although the causes of sparkle are not entirely understood, observation reveals that the magnitude of sparkle may depend on such parameters as the surface texture of the subject coating, the distance between the subject film and underling components of the optical device (e.g., the pixels of a display), and the size of the display pixels. A problematic trend is that sparkle tends to increases as pixel size of an underlying display decreases. Thus displays in which display resolution is increased by reducing the size of constituent pixels tend to exhibit problematic sparkle.
The need exists for improved matte coats that provide desired anti-glare performance while also providing desired low sparkle.